The subject of the invention is a method for the identification of weak and/or strong branches of an electric power transmission system comprising at least one generator and nodes, interconnected by transmission lines, useful especially for the determination of the weak branches of the analyzed system. The method for the determination of the weak branches of a power system employs known methods of determining the voltage stability of the whole system and predicts the voltage stability margin in specific branches of the power system.
From U.S. Pat. No. 5,745,368 there is known a method of voltage stability analysis in electric power systems. That description discloses a method which is appropriate for low and high voltage applications as well as differing types of loads and load variations. In that method, a nose point of a P-Q curve showing functional relation between voltage and power is found, from which the distances to points characterizing the reactive, active and apparent power are calculated, while a generalised curve fit is used to compute the equivalent or surrogate nose point. The determination of that point is achieved by approximating a stable branch and creating a voltage versus power curve, determining a plurality of stable equilibrium points on the voltage and load curve, using the plurality of determined stable equilibrium points to create and fit an approximate stable branch, calculating an approximate voltage collapse point and thereafter a voltage collapse index. The value of that index allows for predicting the occurrence of expected voltage collapse under specific conditions.
From a European patent application No. EP 1 134 867 there is known a method for the assessment of stability of electric power transmission networks. The method comprises the measurement of vector quantities for voltage and current in numerous points in the network, transfer of those data to the system protection center, transfer of information regarding the operating condition of equipment in the substations of that network, and, on the basis of the acquired data, determination of at least one stability margin value of the transmission network. The measured vectors may be represented by quantities such as voltage, current, power, or energy connected with phase conductor or an electronic system.
The method for the identification of weak and/or strong branches of an electric power system according to the invention can be possibly employed as a useful solution for the assessment of stability of power networks, for example, in the solution presented in the application EP 1 134 867, although the identification of weak branches in networks is made apart from the methods of network stability assessment as presented in the state of the art, and the method as such is not yet known.
On the other hand, from U.S. Pat. No. 5,796,628 there is known a dynamic method for preventing voltage collapse in electric power systems. In the presented solution xe2x80x9cweak areasxe2x80x9d in networks are identified. These areas are defined as those parts of the network which do not withstand additional load. The solution introduced in that description consists in monitoring the power network through the surveillance of real-time data from the network, forecasting the near-term load of each branch of the network as well as the power demand in that branch on the basis of those data, and in order to estimate the system stability, such that each of the branches would be able to withstand the expected load, the amount of the margin of reactive and/or active load is defined. The proposed value of this load as well as the proposed voltage profiles are determined on the basis of the known power flow technique and the saddle-node bifurcation theory.
The method for the identification of weak and/or strong nodes of an electric power transmission system according to the invention, which employs known computational methods regarding power flow in the nodes and branches of the electric power transmission system, and in which functional relations between active and reactive loads for that system are analyzed, consists in subjecting the characteristic electric parameters of nodes and branches of the power system to computational treatment to achieve power flow equations for all that system""s nodes with assumed 100 percent value of the system""s basic load, and calculating complex voltage values in those nodes. Then an electric model of a system branch located between two receiver nodes is assumed, and a limiting curve P-Q showing the functional relation between active load and reactive load for the assumed electric model of the branch is constructed, and a base load point for that branch is assumed. Then the branch coefficient of voltage stability is determined for the analyzed system branch, thereafter the total system load is increased to overload the system up to 120% of the base load, and all steps relating to the determination of the voltage stability coefficient for the analyzed branch at the predetermined overload of the system are repeated. The numerical value of the voltage stability coefficient is compared with the threshold value considered to be a safe margin for maintaining voltage stability in the analyzed branch, the value of the difference between the values of the branch voltage stability coefficients determined for both types of system load is checked, i.e. whether it is more than, equal to, or less than zero, and on the basis of those comparisons the analyzed branch is identified as weak or strong.
The branch coefficient of voltage stability is preferably calculated from the following relation:
cvc=dvcxc2x7(1xe2x88x92pvc), 
where:
dvc=√{overscore (pcrxe2x88x92pb)2+(qcrxe2x88x92qb)2)}{overscore (pcrxe2x88x92pb)2+(qcrxe2x88x92qb)2)}xe2x80x94is the distance between the base point of branch load and the critical point on the P-Q curve, and       p    vc    =      1    -          xe2x80x83        ⁢                                                                      ∫                                  P                  min                                                                                            -                                                                        Q                          min                                                                          1                          -                                                                                                                    X                                b                                                            ⁢                                                              B                                b                                                                                      2                                                                                                                +                                          0.25                                                                        (                                                      1                            -                                                                                                                            X                                  b                                                                ⁢                                                                  B                                  b                                                                                            2                                                                                )                                                2                                                                                                        ⁢                              (                                                                            -                                              (                                                  1                          -                                                                                                                    X                                b                                                            ⁢                                                              B                                b                                                                                      2                                                                          )                                                              ⁢                                          P                      2                                                        +                                                                                                                                                              +                                          0.25                                              1                        -                                                                                                            X                              b                                                        ⁢                                                          B                              b                                                                                2                                                                                                      -                                      Q                    min                                                  )                            ⁢                              xe2x80x83                            ⁢              d              ⁢                              xe2x80x83                            ⁢              P                                                            (                                    P              max                        -                          P              min                                )                ·                  (                                    Q              max                        -                          Q              min                                )                    
xe2x80x94is the probability of
occurrence of voltage instability in the analyzed branch.
Preferably, the analyzed branch is considered weak where the value of the branch voltage stability coefficient for 100% system load is less than 0.125 and at the same time the difference between the coefficient determined for the given node at total system load equal to 100% and the coefficient determined for the given node at total system load equal to 120% is more than zero, or the analyzed branch is considered strong where the value of the branch voltage stability coefficient for 100% system load is less than 0.125, the difference between the coefficient determined for the given node at total system load equal to 100% and the coefficient determined for the given node at total system load equal to 120% being less than or equal to zero.
The advantage of the inventive method is the ability to determine weak and/or strong branches of an electric power transmission system without the need for making a multivariant analysis of power flow in the power system considering the critical loads and cutouts of individual system elements.